Atomic spectrometry detection frequently requires the ready availability of a liquid sample. Conventional sample introduction techniques for atomic spectrometry detection rely predominantly on pneumatic nebulization of liquids.
There are several techniques in current use for vapor generation, but this is classically accomplished using chemical derivatization reactions which are conducted in separate modules and frequently independent of the sample nebulization process. The most popular of these techniques is the so called hydride generation approach, which relies on the reductive hydridization of a small number of elements by the action of an aqueous solution of sodium tetrahydroborate. This approach, as well as others relating to halide generation and aqueous alkylation reactions for generation of volatile slightly water soluble forms of metals is discussed in R. E. Sturgeon and Z. Mester, Analytical Applications of Volatile Metal Derivatives, Appl. Spectrosc. 56 202A-213A (2002).
These metal vapour generation protocols are limited in scope to a handful of elements and are themselves difficult to implement, frequently requiring separate gas-liquid separators and excluding all other elements not amenable to the derivatization reaction.
Enhancement of sample introduction efficiency is currently being pursued by many practitioners of atomic spectrometry. Current activity includes the design of improved nebulizers and spray chambers, frequently operating at low sample uptake and ultimately relying on their integration or complete elimination of the latter so as to achieve 100% efficiency or utilizing chemical vapor generation (CVG) to convert the analytes of interest to volatile species, thereby achieving similar results. CVG is undergoing a resurgence of interest in the past decade following the report of a volatile species of copper generated during merging of an acidified solution of the analyte with that of sodium tetrahydroborate reductant. Subsequently, a number of transition and noble metals have been detected based on similar reactions, but typically under conditions facilitating rapid separation of the relatively unstable product species from the liquid phase. This requirement is most easily met when the sample and reductant solutions are merged at the end of a concentric or cross-flow nebulizer, the resultant aerosol providing a unique atmosphere for rapid release of the volatile product from a large surface-to-volume phase into an inert transport gas.
A simplified and potentially “cleaner” arrangement for vapor generation can be realized with the use of ultraviolet irradiation. See, for example, X. Guo, R. E. Sturgeon, Z. Mester and G. J. Gardner, UV Vapor Generation for Determination of Se by Heated Quartz Tube AAS, Anal. Chem. 75 2092-2099 (2003). Although UV has been widely deployed to assist with oxidative sample preparation, its application as a tool for alkylation of a number of metals has only recently emerged. Radical induced reactions in irradiated solutions of low molecular weight organic acids provide small ligands capable of reducing, hydrogenating and/or alkylating a number of elements to yield volatile products. X. M. Guo, R. E. Sturgeon, Z. Mester and G. J. Gardner, Anal. Chem., 2004, 76, 2401-2405.
To date, the process of photoalkylation for analytical purposes (enhanced detection capability for metals, semi-metals or non-metals) has been achieved using either one of two approaches: irradiation of sample in a batch reactor containing the analyte element of interest and the LMW acid which is connected to analytical instrumentation used for element detection via a gas transport line; or by irradiation of a continuous flowing stream of sample containing the analyte element of interest and the LMW acid which is directed to a gas-liquid separator for phase separation and transport of a carrier gas containing the generated analyte to the detection system. These techniques are not, however, suitable for efficient sample preparation for atomic spectrometry equipment.